Lost, say, a cult TV series (8)
I believe the answer is:
casualty
'tv series' is the definition.
(medical drama)
'lost say a cult' is the wordplay.
'lost' indicates anagramming the letters.
'say'+'a'+'cult'='sayacult'
'sayacult' with letters rearranged gives 'CASUALTY'.
(Other definitions for casualty that I've seen before include "TV hospital drama" , "Injured party" , "Wounded victim" , "Hospital department" , "Victim of accident or war" .)